This invention relates to telecommunication networks, and more particularly to a bus-oriented asynchronous transfer mode (ATM) communication network used in a LAN (local area network) configuration.
Asynchronous transfer mode (ATM) is a communications technology on which messages from a large number of sources are concentrated into a single path by asynchronous time division multiplexing. The communications medium itself is synchronous, in that bits are sent at a fixed speed, often within frames; cells are sent within frames, and octets within cells. From the standpoint of a given port, communications are asynchronous in that information in the form of cells is sent on demand and the cells assigned to transfer a message may be irregularly ordered without regard to position within frames. In ATM systems, a port or station in the network may be a concentrator, a single-use or multiple-use computer, or another communicating device.
Local area networks (LANs) for communications among a few dozen or a few hundred desktop stations have not previously made use of ATM methods. Instead, LANs are usually of the asynchronous type, in that messages are sent by a station upon demand, and a regular ordered communication path is not continuously maintained. The technologies most used for LANs are Ethernet, token ring, FDDI, at the like.
Traditional LAN technology is distinctive from traditional wide-area network (WAN) technology used for nationwide or global communications networks. LANs operate asynchronously, but data needs to be converted to a different format for transmission across the WAN, which may or may not use ATM techniques. Traffic concentrators with suitable conversion facilities are of course available.
In order to provide a more seamless match between a local network and a wide area network, it may thus be appropriate to use an ATM type of communications system as a LAN. Also, it is expected that the demand for higher levels of performance in LANs will dictate an ATM type of technology for local systems. In adapting ATM technology for use as a LAN, a switching architecture is required that economically provides for many high-speed ports. Large crossbars or other matrices, whose cost per port rises relatively quickly (as the size of a matrix grows more rapidly than the number of ports), are an expensive option. While a central office-oriented ATM switch must cope with aggregate traffic rates that reflect a relatively high average utilization of each interface, an ATM switch aimed at providing the primary LAN service to many end-user desktops need merely provide an aggregate bandwidth equal to the total demand from every user of the switch, averaged over an acceptable latency.
Accordingly, an objective is to provide an asynchronous communication method for a local network, at relatively low cost yet high performance, so that an interface with ATM networks is facilitated.